Chlamydial disease of humans includes predominant ocular, genital and respiratory tract infections, with sequelae ranging from blindness, to female infertility, arthritis and asthma. Chronic infection with the respiratory pathogen, Chlamydia pneumoniae is also associated with coronary heart disease, the number one killer disease of humans. In spite of their public health magnitude, chlamydiae are reputed for their elusiveness as infectious microorganisms to clinicians and molecular biologists alike. This owes to several factors, prominent among which are a unique obligate intracellular developmental lifestyle and the fact that chlamydiae have resisted genetic manipulation to this day. We have isolated a bacteriophage, phiCPG1 from the model Chlamydia psittaci strain ?Guinea Pig Inclusion Conjunctivitis?. A member of the single-stranded DNA microviridae family, phiCPG1 is nearly identical to a ?virtual? phage of C. pneumoniae that was revealed by genome sequence analysis. The infection of an intracellular pathogen by its own parasitic bacteriophage is a unique biological phenomenon, with potentially important implications in infection and disease. Moreover, phages offer unique opportunities for the development of molecular and genetic tools for research. The objectives of this application are therefore to gain a broad understanding of Chlamydia phage biology in the context of chlamydial infection. We will determine the molecular basis of the interaction of the phage with its host and comparatively evaluate gene expression in phage-free and phage-infected bacteria. The availability of well-established models of infection and disease in the guinea pig will allow for the first time to study the impact of phage infection on the natural infection of a vertebrate animal by an obligate intracellular pathogen. Finally, the information gained in these studies will be exploited toward the development of genetic methodologies in Chlamydia.